Reactions of Acids with Naphthyridine-Functionalized Ferrocenes

Article pubs.acs.org/IC

Reactions of Acids with Naphthyridine-Functionalized Ferrocenes: Protonation and Metal Extrusion Nabanita Sadhukhan,† Mithun Sarkar,† Tapas Ghatak,† S. M. Wahidur Rahaman,† Leonard J. Barbour,‡ and Jitendra K. Bera*,† †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Department of Chemistry, University of Stellenbosch, 7602, Stellenbosch, South Africa

‡

S Supporting Information *

ABSTRACT: Reaction of 1,8-naphthyrid-2-yl-ferrocene (FcNP) with a variety of acids affords protonated salts at first, whereas longer reaction time leads to partial demetalation of FcNP resulting in a series of Fe complexes. The corresponding salts [FcNP·H][X] (X = BF4 or CF3SO3 (1)) are isolated for HBF4 and CF3SO3H. Reaction of FcNP with equimolar amount of CF 3 CO 2 H for 12 h affords a neutral complex [Fe(FcNP)2(O2CCF3)2(OH2)2] (2). Use of excess acid gave a trinuclear FeII complex [Fe3(H2O)2(O2CCF3)8(FcNP·H)2] (3). Three linear iron atoms are held together by four bridging trifluoroacetates and two aqua ligands in a symmetric fashion. Reaction with ethereal solution o f H Cl afforded [(FcNP·H)3(Cl)][FeCl4]2 (4) irrespective of the amount of the acid used. Even the picric acid (HPic) led to metal extrusion giving rise to [Fe2(Cl)2(FcNP)2(Pic)2] (5) when crystallized from dichloromethane. Metal extrusion was also observed for CF3SO3H, but an analytically pure compound could not be isolated. The demetalation reaction proceeds with an initial proton attack to the distal nitrogen of the NP unit. Subsequently, coordination of the conjugate base to the electrophilic Fe facilitates the release of Cp rings from metal. The conjugate base plays an important role in the demetalation process and favors the isolation of the Fe complex as well. The 1,1′-bis(1,8-naphthyrid-2-yl)ferrocene (FcNP2) does not undergo demetalation under identical conditions. Two NP units share one positive charge causing the Fe-Cp bonds weakened to an extent that is not sufficient for demetalation. X-ray structure of the monoprotonated FcNP2 reveals a discrete dimer [(FcNP2·H)]2[OTf]2 (6) supported by two N−H···N hydrogen bonds. Crystal packing and dispersive forces associated with intra- and intermolecular π−π stacking interactions (NP···NP and Cp···NP) allow the formation of the dimer in the solid-state. The protonation and demetalation reactions of FcNP and FcNP2 with a variety of acids are reported.

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INTRODUCTION Protonation of ferrocene (Fc), the simplest model for the electrophilic substitution of metallocene, has been the subject of numerous experimental and computational investigations.1 The proton affinity of ferrocene is well recognized, but the precise site of proton attack remains an unresolved issue. NMR studies of Fc in aqueous BF3 suggested a metal-hydride species in solution.2 Subsequent experiments indicated a mixture of metal-protonated, agostic (endo), and ring-protonated (exo) species in deuterated medium (Scheme 1).3 It has been proposed that the hardness of an electrophile determines the site of attack. A hard electrophile like acetyl cation (CH3CO+) prefers the ring-attack (exo) whereas mercuration by soft Hg2+ follows the metal-attack (endo).4 For proton, a rapid equilibrium between metal-protonated and agostic forms is observed.5 Thus, a clear preference for metal or ring-attack could not be established from the experimental data. Computational studies have afforded a set of contradicting conclusions. At the HF level, ring-protonation is favored over metal© 2013 American Chemical Society

Scheme 1. Metal-Protonated (a), Agostic (b), and RingProtonated (c) Forms of Protonated Ferrocene

protonation. Inclusion of electron correlation at the MP2 level, however, caused a reversal in the preferred site.6 Calculations at CCSD(T) level of theory affords minima for metal-protonated Received: October 3, 2012 Published: January 24, 2013 1432

dx.doi.org/10.1021/ic302155e | Inorg. Chem. 2013, 52, 1432−1442

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and agostic forms, though no energy-minima structure was identified for ring-protonated form.7 Interaction of Li+ with ferrocene revealed a stable structure where Li+ was located on top of the π-face of the Cp ring.8a,b A lateral bound Li+ to the iron atom of the ferrocene has also been reported.8c The Ga+ interacts with ferrocene on top of the Cp ring in a multidecker complex consisting of alternate Ga+ and ferrocene.8d Intramolecular protonation of Fe by NH is reported in a ferrocenophane with bridging guanidines.9a Protonation by oxoanion HSO4− is credited for the remarkable fluorescence enhancement of a ferrocene-based ion pair receptor cobound with PbII or ZnII.9b Acylferrocene (FcCOR) and ferrocenyl ethers [Fc(CH2)nOR] are protonated at the carbonyl oxygen and ether oxygen, respectively, providing stable protonated species.10 Density functional theory (DFT) studies on protonated formylferrocene show that the oxygen protonated isomer is stable compared to ring or metal-protonated species.11 Metal-ion coordination to functionalized ferrocenes has produced a rich chemistry providing access to myriad of heterobimetallic complexes.12 In general, the metal ion binds the donor atom of the ferrocene appendage. Careful examination has revealed additional interactions which are worth mentioning. A Ni−Fe heterobimetallic complex featuring Fe→Ni dative bond has been reported with ferrocenedithiolato ligand (Scheme 2a).13a Similarly, Fe···Zr,13b Fe···Pd,13c,d

Scheme 3. Line Drawings of FcNP and FcNP2

cycle comprising two head-on ligands bridged by two metal ions. A similar macrocycle supported by intermolecular hydrogen bonding (H-bonding) interactions is obtained upon protonation of FcNP2. This initial result prompted us to explore other protonated ferrocene-naphthyridine hybrids which eventually lead to a wider investigation. While protonated salts were isolated for HBF4 and CF3SO3H, partial release of metal from FcNP occurred upon reactions with CF3COOH, HCl, and picric acid resulting in different Fe complexes containing the corresponding conjugate anions. FcNP2 does not undergo demetalation under identical condition, and affords both mono- and bis-protonated adducts. Protonation of FcNP. Reactions with CF3SO3H and HBF4. Reaction of triflic acid to a methanolic solution of FcNP in 1:1 molar ratio for 1 h afforded a purple solid [FcNP·H][OTf] (1). Triflate anion is found to be H-bonded with the N−H hydrogen through one of the oxygen atoms (Supporting Information, Figure S1). The NP unit is planar with the Cp ring (inclination angle: ∠N2−C8−C9−C10 = 4.4(3)°). Reaction with HBF4 provided an analogous protonated salt [FcNP·H][BF4] which shows protonation at N8 and its H-bonding interaction with one of the F atoms of BF4−.18 Reaction with Equimolar CF3CO2H. Reaction of CF3CO2H with FcNP in 1:1 molar ratio for 12 h afforded a neutral complex [Fe(FcNP)2(O2CCF3)2(OH2)2] (2). The most remarkable feature of this complex is the central iron which originates from FcNP. The molecular structure reveals two FcNP, two trifluoroacetates, and two water molecules coordinated to Fe in mutually trans orientations creating an octahedral geometry around the metal (Figure 1). Each FcNP exhibits preferential κ1-binding utilizing N8 of the NP unit. The Fe1−N1 distance is 2.271(2) Å. The Fe1−O1W (aqua) distance is 2.132 (2) Å, and the Fe1−O2A (carboxylate) distance is 2.125 (2) Å. The cis angles are in the range 84.54(7)°−95.46(7)°. Reaction with Excess CF3CO2H. Reaction of trifluoroacetic acid to a methanolic solution of FcNP in excess (10:1 molar ratio) for 12 h afforded a neutral, linear trinuclear FeII compound [Fe3(μ-H2O)2(μ-O2CCF3)4(κ1O-O2CCF3)4(κ1N8FcNP·H)2] (3) in high yield (70%). The molecular structure, as shown in Figure 2, consists of three iron atoms forming a linear array (∠Fe···Fe···Fe = 180°). The central iron resides on a crystallographically imposed inversion center. Three iron atoms are held together by four bridging trifluoroacetates and two bridging aqua ligands in a symmetric fashion. Each terminal iron is additionally coordinated to two monodentate trifluoroacetates. The trinuclear core is capped by protonated FcNP on both sides limiting the formation of a higher order oligomer. The distal nitrogen (N8) in FcNP is utilized for metal coordination whereas the proximal nitrogen (N1) is found to be protonated. Engagement of both N atoms of FcNP to metal

Scheme 2. Heterobimetallic Complexes of Ferrocene Based Ligands Showing Fe···M and M···Cp Interactions

Fe···Zn,13e and Ru···Cu/Zn13f interactions are reported in complexes containing metallocene based ligands. Reaction of ZnCl2 with 1,1′-bis(quinolyl)metallocene (Fc*Qu) afforded [ZnCl2(Fc*Qu)] which exhibits interaction of Zn with carbon atom of the Cp ring (Scheme 2b).14 Reductive degradation of ferrocene was reported long back by the application of Li in methylamine to metallic iron and cyclopentadiene.15 Interaction of Lewis acid BCl3 with the functionalized Cp ring in Fc*Qu led to Fe extrusion from the ligand affording an organoborane compound.14 Molina and coworkers have reported metal extrusion from a bis-ferrocenylazaheterocycle on treatment with HBF4 or HCl.16 Herein, we disclose the protonation and metal extrusion of 1,8-naphthyrid2-yl-ferrocene (FcNP) and 1,1′-bis(1,8-naphthyrid-2-yl)ferrocene (FcNP2) (Scheme 3) with a variety of acids.

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RESULTS AND DISCUSSIONS The coordination chemistry of naphthyridine-functionalized ferrocene derivatives FcNP and FcNP2 is described in a recent report.17 Reaction of FcNP2 with CuI afforded a metallamacro1433


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Binuclear iron complexes bearing carboxylate ligands have attracted considerable attentions because of their relevance in nonheme iron enzymes.19 The active site of one such enzyme, soluble methane monooxygenase (MMO) which converts methane to methanol by using O2, contains a diiron core wrapped by four glutamate carboxylates and two histidine imidazoles (Scheme 4).20 Metal-bound water molecules, further Scheme 4. Active Site of Non-Heme Iron Enzyme

Figure 1. ORTEP diagram (40% probability thermal ellipsoids) of 2 with important atoms labeled. Carbon atoms of the ferrocene units are shown as circles of arbitrary radius. Hydrogen atoms are omitted for the sake of clarity. Selected bond distances (Å) and angles (deg): Fe1−O2A 2.125(2), Fe1−O1W 2.132(2), Fe1−N1 2.271(2), O1W− Fe1−O1W′ 180.00. Symmetry code: 1−X, 1−Y, −Z.

supported by H-bonding interactions with the carboxylates, are recognized to be the key components for oxygen binding and activation.21 This led to a conscious effort for the synthesis of model complexes containing water molecules in the coordination sphere of diiron(II) carboxylate.22 The triangular {Fe3O(O 2 CR) 6 } unit has dominated the chemistry of iron carboxylates for decades.23 We are aware of an example [FeII3(μ-O2CCH3)6(κ2-biphme)2]24 (biphme = 2,2′-(methoxy(phenyl)methylene)bis(1-methyl-1H-imidazole)) which contains a linear Fe3 core supported by only the acetates (Fe···Fe = 3.3251(9) Å) (Scheme 5). Complex 3 is the only example of a linear trinuclear iron complex with bridging aqua and carboxylate ligands. Reaction with HCl. Reaction of 1(M) HCl in Et2O with FcNP (1:1 molar ratio) in methanol for 12 h afforded [(FcNP·H)3(Cl)][FeCl4]2 (4). The same product was isolated when excess acid (10-fold) was used. The asymmetric unit consists of three protonated [FcNP·H]+, one chloride, and two

ions was reported earlier for dicopper(I) and dirhodium(II) complexes.17a The Fe···Fe separation is 3.726(1) Å. The Fe−O (carboxylate) distances are in the range 2.049(2)−2.119(2) Å. The Fe1−O1W distance is 2.239(2) Å, and the Fe1−O1W− Fe2 angle is 115.33(10)°. The unbound oxygen atoms of monodentate trifluoroacetates are engaged in H-bonding with the bridged water molecules. The OW−(H)···O distances are 2.644 and 2.687 Å. Each iron center possesses an octahedral geometry revealing a small range of deviation in cis (86.7(1)°− 93.5(1)°) and trans (177.5(1)°−178.9(1)°) angles. The spectroscopic and electrochemical data of the crude solid do not match with that of pure 3 indicating the existence of other products. It is possible that several oligomeric complexes are formed in solution. Neutral compound 3 separates out from the solution in form of crystals probably because of its lesser solubility.

Figure 2. ORTEP diagram (40% probability thermal ellipsoids) of 3 with important atoms labeled. Carbon atoms of the ferrocene units are shown as circles of arbitrary radius. Hydrogen atoms except water and N−H are omitted for the sake of clarity. Selected bond distances (Å) and angles (deg): Fe1···Fe2 3.726(1), Fe1−O1W 2.239(2), Fe2−O1W 2.171(2), Fe1−N1 2.224(2), Fe1−O7 2.119(2), Fe1−O4 2.099(2), Fe1−O2 2.094(2), O1W− (H1W)···O1 2.687(3), O1W−(H2W)···O3 2.644(4). O4−Fe1−O1W 91.56(9), O5−Fe1−N1 91.40(9), O5−Fe1−O1W 90.61(9), O8−Fe2− O1W′ 91.71(8), Fe2−O1W−Fe1 115.33(10), O2−Fe1−O1W 90.48(8), Symmetry code: 1−X,−Y, 1−Z. 1434


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Scheme 5. Examples of Linear Trinuclear Fe(II) Complexes

[FeCl4]−. X-ray structure reveals protonation at the N8 atom of NP, and all three NH hydrogens are engaged in H-bonding interactions with the central chloride in pseudo C3 symmetry (Supporting Information, Figure S2). As in previous cases, the source of iron in [FeCl4]− is FcNP. It is likely that the bivalent iron that is extracted from ferrocene is oxidized to FeIII by the adventitious oxygen. Reaction with Picric Acid. Prompted by the metal extrusion from FcNP by certain acids, we wondered if the same could be achieved with an organic acid. Reaction of FcNP with 1 equiv of picric acid (HPic) in methanol for 12 h led to an immediate color change from orange red to green. Subsequent crystallization of the product from dichloromethane/petroleum ether afforded a neutral compound [Fe2(μ-Cl)2(FcNP)2(Pic)2] (5). Use of a mixture of acetonitrile/diethyl ether as crystallizing solvents afforded chloride-free neutral Fe(FcNP)(Pic)2 which was characterized spectroscopically.25 This suggests that the source of chlorides in 5 is the dichloromethane employed during crystallization. We could not isolate a clean identifiable product when excess acid was used. The Xray structure of 5 reveals partial demetalation resulting in a dinuclear FeII complex bridged by two chlorides. The origin of iron is FcNP as in previous cases. Complex 5 possesses a crystallographically imposed C2 symmetry, and only half of the molecule is observed in the asymmetric unit (Figure 3). The geometry around each iron is pseudo octahedral consisting of one bidentate chelate picrate, one chelate FcNP, and two bridging chlorides. The Fe···Fe distance is 3.430(1) Å. The naphthyridine chelates the metal ion with bite angle 59.97(11)°.26 The Fe−Cl distance and Fe−Cl−Fe angle are 2.4201(10) Å and 90.05(3)°, respectively. The picrate anion utilizes oxygen atoms of the phenoxy and one of the ortho-nitro groups to coordinate with the metal. Notable is the difference in Fe−O distances, the nitro oxygen makes longer Fe−O distance (2.226(2) Å) compared to phenoxy oxygen (1.982(2) Å). The cis (59.97(11)°−106.46(10)°) and trans angles (153.85(8)−174.45(7)°) reveal severe distortions from the ideal octahedral geometry around the central iron atom. Transition metal-picrate complexes are relatively rare although picrates have been used widely in lanthanide chemistry.27 A Cambridge Crystallographic Data Centre (CCDC) search revealed several complexes with group 11 metals Cu and Ag.28 Examples of MnII complexes incorporating picrate ligands are known in the literature.29 It appears that complex 5 is the only example of an iron complex containing picrate ligands mounted on a doubly chloro-bridged Fe2Cl2 core.

Figure 3. ORTEP diagram (50% probability thermal ellipsoids) of 5 with important atoms labeled. Carbon atoms of the ferrocene units are shown as circles of arbitrary radius. Hydrogen atoms are omitted for the sake of clarity. Selected bond distances (Å) and angles (deg): Fe1···Fe1′ 3.429 (1), Fe1−N1 2.129(3), Fe1−N2 2.368(3), Fe1−O1 2.226(2), Fe1−O3 1.982(2), Fe1−Cl1 2.420(10), Fe1−Cl1′ 2.432(1), Fe1−Cl1−Fe1′ 89.95(3), O3−Fe1−Cl1 95.60(7), N1−Fe1−Cl1 102.70(8), O3−Fe1−O1 78.99(10), N1−Fe1−N2 59.97(11), N2− Fe1−Cl1 94.36(7), Cl1−Fe1−Cl1′ 90.05(3). Symmetry code: 1−X, 1−Y, 1−Z.

Protonation of FcNP2. Protonation of FcNP2 with one or more equivalents of triflic acid afforded red-colored needleshaped crystals of molecular formula [(FcNP2·H)]2[OTf]2 (6) in high yield. X-ray structure reveals a discrete H-bonded dimer reminiscent of [Cu2(FcNP2)2]2+ metallamacrocycle reported earlier by us.17a Molecular structure shows two [FcNP2·H]+ units in head-on arrangement, linked by intermolecular N− H···N hydrogen bonds (Figure 4). Only half of the dimer was observed in the asymmetric unit related to the other half by a center of inversion. The N2···(H4)−N4 distance is 2.705(4) Å and N2−H4−N4 angle is 169.73(5)°. The rectangular shaped H-bonded dimer 6 features a box width of 11.366(1) Å (Fe···Fe separation) comparable to [Cu2(FcNP2)2]2+ (Fe···Fe = 11.104(3) Å). 1435


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Figure 4. ORTEP diagram (40% probability thermal ellipsoids) of the dicationic unit [(FcNP2·H)2]2+ in 6 with important atoms labeled. Carbon atoms of the ferrocene units are shown as circles of arbitrary radius. Hydrogen atoms except N−H are omitted for the sake of clarity. Selected bond distances (Å) and angles (deg): N4−(H4)···N2 2.705(4). N4−H4−N2 169.73(5). Symmetry code: 1−X, −Y, −Z.

The NP units in free FcNP2 are in a trans conformation.17b Protonation leads to 180° rotation along the C5 axis resulting in a cis geometry stabilized through H-bonding interactions with the second molecule. Similar observation is reported for [Fc(CO2)2]2− which exhibits a trans conformation like FcNP2. Protonation leads to a syn chelate geometry for [Fc(CO2)(CO2H)]− which acquire thermodynamic stability via H-bond interactions between −CO2− and −CO2H units.30 The mother liquor obtained after isolation of 6 was kept in a refrigerator, and a small crop of crystals of different morphology (block-shaped) was harvested after 3 weeks. X-ray analysis confirmed it as the bis-protonated salts [FcNP2·H2][CF3SO3]2 (7). Molecular structure reveals protonation at the N8 atom of both NP units. Further, the N−H hydrogens are engaged in Hbonding interactions with two independent triflates (Figure 5). The Cp and NP planes are planar and NP nitrogens are disposed in opposite directions. This is in sharp contrast to the NP arrangement in monoprotonated complex 6. Precedence of a double protonated structure [FcQu2·H2]2+ (FcQu2 = 1,1′bis(8-quinolyl)ferrocene) exists in the literature.13b

Cyclic Voltammetry Studies. The electrochemical properties of compounds 1−6 are studied by cyclic voltammetry in propylene carbonate, and the redox potential values are collected in Table 1. Substitution of the Cp ring with a NP Table 1. Electrochemical Potentials (in Volt) from Cyclic Voltammetry for FcNP, FcNP2, and 1−6 in Propylene Carbonate compounds

oxidation

reduction

FcNP FcNP2 1 2 3 4 5 6

0.53(180)a 0.66(100)a 0.64(140)a 0.59(120)a, 0.48b 0.57(70)a, 0.15(170)a 0.64(120)a 0.64(110)a, 0.06(74)a 0.93(111)a

−1.93c −1.87c −1.59c −1.93c −0.78c −0.48c, −1.66c −1.89c −0.49c, −0.80c

a

Half-wave potentials evaluated from cyclic voltammetry as E1/2 = (Ep,a + Ep,c)/2, peak potential differences in mV in parentheses. bPeak potentials, Ep,a for irreversible oxidation processes. cPeak potentials, Ep,c, for irreversible reduction processes.

unit brings about a shift of 60 mV for ferrocenyl oxidation with respect to ferrocene (E1/2 for Fc = 0.47(60) V under identical experimental conditions). The second substitution causes further positive shift of the oxidation by 130 mV. Modulation of redox potential by Cp ring substitution with pyridyl and pyrimidyl groups has been reported, and the values observed for naphthyridine derivatives are in close agreement.31 Protonation causes an anodic shift of 110 mV of Fc+/Fc couple with respect to FcNP reflecting a significant charge transfer from ferrocenyl donor to NP based acceptor. The NP based reduction appears at Ep,c = −1.59 V indicating easier reduction of the protonated NP unit. Complex 2 exhibits ferrocenyl oxidation at E1/2 = 0.59(120) V which is 50 mV lower than that of the protonated complex 1. The irreversible oxidation of the central iron appears at Ep,a = 0.48 V. Although complex 3 contains three different Fe-based moieties, namely, protonated FcNP, two terminal Fe, and a central Fe of the trinuclear core, only two oxidation processes at 0.15(170) V and 0.57(70) V are observed. Although the first oxidation can safely be assigned to the Fe3 core, the same is not so obvious for the second oxidation. It is unlikely that the metalcoordinated FcNP.H+(N1) would oxidize at such a low potential since the N8 protonated ligand oxidizes at 0.64 V in

Figure 5. ORTEP diagram of (50% probability thermal ellipsoids) 7 with important atoms labeled. Carbon atoms of the ferrocene units are shown as circles of arbitrary radius. Hydrogen atoms except N−H are omitted for the sake of clarity. Selected bond distances (Å) and angles (deg): N1−(H1)···O1A 2.732(5), N3−(H3)···O4A 2.718(5) N1− H1−O1A 161.44(6), N3−H3−O4A 155.21(5). 1436


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away from the metal as evidenced by the increase in oxidation potential (see Table 1). The low-lying Cp-NP π* orbitals are stabilized upon protonation resulting in red-shifting of the MLCT transitions. Further, increased mixing of the metal orbitals with appropriate Cp-NP orbitals enhances the MLCT character of a transition and thus causes an increase in the absorption intensity.33 Figure 7 compares the absorption

1. Simultaneous protonation at proximal nitrogen (N1) and metal-coordination through N8 should make the FcNP highly electron deficient, which is reflected in the low reduction potential at Ep,c = −0.78 V. Accordingly, we attribute both these oxidation processes to the Fe3 core. The FcNP·H+ in 3 does not appear to exhibit oxidation within the accessible range of this experiment. The Fc-based oxidation potential in 4 is identical to 1. It is understandable since both complexes essentially contain the protonated FcNP. Complex 4 exhibits an irreversible reduction at Ep,c = −0.48 V which is attributed to the anionic [FeIIICl4] core. Two reversible oxidation processes are recorded for compound 5 at E1/2 = 0.64(110) and 0.06(74) V which correspond to FeIII/FeII couples for FcNP and Fe2Cl2 core, respectively (Figure 6). The ferrocene based oxidation in

Figure 7. Electronic absorption spectra for FcNP (red line) and 1 (blue line) in dichloromethane at room temperature.

spectra of FcNP and its protonated complex 1. On protonation, the d-d and MLCT bands of FcNP are significantly red-shifted accompanied by increase in molar absorption coefficient values (Table 2). Protonated FcNP units are characterized by a lowenergy (∼600 nm) broad MLCT band as observed in complexes 1, 3, 4, and 6. Further on the Structure of 6. The association of two positively charged protonated FcNP2 in 6 is intriguing. Since the positions of hydrogens could not be ascertained accurately from X-ray diffraction, we carried out density functional theory (DFT) calculations. The optimized structure of the dimer [(FcNP2·H)+]2 (B97D/6-311++G(d,p) (C,H,N)/LANL2DZ (Fe)) closely resembles the X-ray geometry. Use of a functional incorporating dispersion correction (B97D)34 afforded planar arrangement of the NP rings in the dimer (Figure 8). This is in contrast to the twisted nonplanar configuration of the NP rings at the B3LYP level of calculation (Supporting Information, Figure S3). It is apparent that dispersive forces are critical to reproduce the X-ray geometry for the protonated dimer. The protons are nearly equidistant from the nitrogens (H1···N3/N4 = 1.269/1.264 Å). Careful examinations revealed that two protons are not exactly on top of each other alleviating the unfavorable repulsion. The H1 makes a shorter contact with N6 (2.899 Å) than H2 (3.196 Å). The same is true for H2 (H2···N3 = 2.899 Å). Other than the in-plane N···H contacts, the perpendicular H···N interactions play a role in overcoming the Coulombic repulsion between two protonated FcNP2. Intramolecular π(NP)−π(NP) (3.592(1) Å) and intermolecular π(Cp)−π(NP) (3.638(3)Å) interactions (Supporting Information, Figure S4) are the key driving forces for the stacked head-on dimer in the solid-state. Complex 6 is sufficiently soluble in common organic solvent which allowed for spectroscopic analysis. All protons in FcNP2 and its protonated form 6 are unambiguously assigned by 1H NMR (Supporting Information, Figures S5, S6) and 1H−1H COSY (Supporting Information, Figure S7, S8) spectroscopy.

Figure 6. Cyclic voltammogram of 5 measured at scan rate of 100 mV/s in propylene carbonate.

6 appears at E1/2 = 0.93(111) V which is 270 mV shifted toward positive potential with respect to FcNP2. The reduction region consists of two irreversible waves at −0.49 and −0.80 V. Electronic Spectroscopy. The electronic spectra of FcNP, FcNP2, and compounds 1−6 were recorded in dichloromethane. The λmax values with the corresponding log ε (ε = dm3 mol−1 cm−1) values are collected in Table 2. The electronic Table 2. Electronic Absorption Data for FcNP, FcNP2, and Compounds 1−6 in Dichloromethane Solvent at Room Temperature absorption spectra compounds

λmax, nm (log ε)

FcNP FcNP2 1 2 3 4 5 6

245 (4.89), 317 (4.49), 388 (3.82), 473 (3.68) 234 (4.59), 314(4.29), 390 (3.68), 503 (3.11) 247(4.67), 361(4.83), 604(4.22) 248 (3.81), 317(3.37), 384(3.12), 467 (2.75) 241 (5.45), 316 (5.30), 384(4.77), 474(4.56), 594 (2.17) 240 (5.00), 320(4.75), 362(4.68), 597 (3.85) 237 (5.00), 327 (5.06), 466(2.11) 248 (3.46), 345(3.32), 590(2.43)

spectra display high energy bands below 250 nm which are assigned to π→π* transitions localized on the Cp-NP units.32 Low energy absorptions are attributed to the ferrocene based spin-allowed d−d transition and Fe(d) → Cp-NP(π*) metalto-ligand charge-transfer (MLCT) transitions. Protonation at the NP unit lowers the energy of Fe-centered highest occupied molecular orbital (HOMO) by withdrawing electron density 1437


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Figure 8. DFT optimized (B97D/LANL2DZ/6-311++G(d,p)) geometry for [(FcNP2·H)2]2+ (left). Illustration of the in-plane and perpendicular N···H interactions (right).

FcNP2 exhibits five sets of nonequivalent NP protons and two sets of Cp protons (Supporting Information, Table S1). Protonated FcNP2 exhibits a similar pattern in CD3CN; however, all protons are downfield shifted due to protonation. The X-ray structure of 6 reveals short separations (2.170(1), 2.467(1) Å) between Ha and Hα′ (of the second molecule) because of the head-on dimer formation (Supporting Information, Figure S9). We performed NOE experiments to gain insight into solution structure. An intensity enhancement for Hb as well as Hα′ is expected for the dimer structure on irradiation of Ha. Similarly, enhancements for Hβ, He, and Ha′ protons are expected for irradiation at Hα. The absence of any change in the intensity for Hα and Ha while irradiating Ha and Hα, respectively, is not in agreement with a close approach of two protonated FcNP2 in solution (Supporting Information, Figures S10−S12). An intramolecular H-bonded structure for the protonated Fc*Py2 (1,1′-bis(2-pyridyl)octamethylferrocene)35 prompted us to look for an analogous structure of [FcNP2.H]+ monomer. Half of the [FcNP2·H]22+ dimer was subjected to DFT optimization which afforded a chelate structure engaging proton in between two inclined NP units (Supporting Information, Figure S13). The calculated N2···N3 distance is 2.509 Å, N2/N3···H distance of 1.283 Å, and the inclination angles of the NP rings from Cp planes are ∼35°. An alternate protonated FcNP2 monomer with hydrogen bridging between proximal nitrogens (N1) could not be optimized. Invariably it gave back the N8 protonated structure. The dimer was computed to be stable (ΔG = −7.63 kcal/mol) compared to the monomer questioning the existence of the later in solution (see Supporting Information, Table S2).36 Proposed Mechanism. We monitored the protonation/ demetalation reactions of FcNP by NMR spectroscopy. All acids employed in this work afford protonated products at first as evidenced by 1H NMR spectra. For example, the NMR spectrum of 1:1 FcNP/HPic in CD3OD shows sharp signals corresponding to protonated FcNP. The signals start broadening after around 6 h which suggests demetalation of FcNP releasing free FeII in the solution. The same observations were made for other acids except for HBF4 for which only protonated FcNP was observed even after 12 h. The protonated salts are isolated for HBF4 and CF3SO3H. Although all acids caused demetalation barring HBF4, Fe complexes are successfully isolated for CF3COOH, HCl, and HPic. NMR study reveals Fe extrusion for CF3SO3H, but an analytically pure compound could not be obtained. The “free” iron in compounds 2−5 is the result of acidmediated demetalation of FcNP. We offer a tentative mechanism for the Fe extrusion. The reaction proceeds with an initial protonation of the NP nitrogen as evidenced by the

isolation of the protonated salts. The NP protonation drains the electron density of the Cp ring toward the NP unit thereby weakening the Fe-Cp interactions. The conjugate base attacks the electrophilic Fe in [FcNP.H]+ facilitating the release of Cp rings from the metal eventually causing metal extrusion. We made an effort to establish a quantitative correlation between demetalated organic product Cp-NP and “extrudedFe”. A careful experiment involving FcNP and picric acid (HPic) in methanol and subsequent treatment with KOH allowed the isolation of Cp-NP.37 A slightly higher amount of Cp-NP was isolated with respect to Fe(FcNP)(Pic)2 (1.18:1). Ideally this value should be 1:1. We attribute this discrepancy to an incomplete isolation of Fe complex by crystallization. The FcNP2 does not undergo demetalation under identical conditions. Two NP units share one positive charge, and hence weaken the Fe-Cp interaction to an extent that is not sufficient for demetalation. Acid mediated demetalation has been observed only for the monosubstituted ferrocene and not from the disubstituted ferrocene unit in a bis-ferrocenylazaheterocycle.16 Concluding Remarks. Reactions of protonic acids with ferrocene-naphthyridine hybrids FcNP and FcNP2 are described in this work. All acids employed in this work afford protonated FcNP initially. Corresponding salts are isolated for HBF4 or CF3SO3H. Longer reaction time leads to partial metal extrusion for each acid except for HBF4. The (protonated) FcNP and the conjugate base trap the released Fe affording isolable complexes. Two different products 2 and 3 are isolated when FcNP and CF3COOH are employed in 1:1 and 1:10 molar ratio, respectively. Only 4 is isolated with HCl irrespective of the amounts of acid used. Use of excess picric acid did not afford a clean product whereas 5 was isolated for 1:1 FcNP and acid. Metal extrusion was also observed for CF3SO3H from NMR study but an analytically pure compound could not be isolated. All complexes reported in this work are charge-neutral except the simple protonated salts. Longer reaction time (10−12 h) appears to be essential for demetalation. The acid-mediated demetalation proceeds with an initial protonation of the NP nitrogen. Subsequently, the conjugate base attacks the electrophilic Fe facilitating the metal extrusion. The conjugate base plays an important role in the demetalation process and in the isolation of Fe complex as well. The FcNP2 does not undergo demetalation under identical conditions. Two NP units share one positive charge and hence weaken the Fe-Cp interaction to a lesser extent not sufficient for demetalation. X-ray structure of the monoprotonated FcNP2 reveals a discrete dimer [(FcNP2·H)]2[OTf]2 (6) supported by two N−H···N hydrogen bonds. Crystal packing and dispersive forces associated with the intra- and 1438


a

1439

15862 4437 3393 276 1.094

32932 9155 6668 531 1.057 0.0422 R1 = 0.0440 wR2 = 0.1042 R1 = 0.0671 wR2 = 0.1212 R1 = 0.0403 wR2 = 0.0639 R1 = 0.0926 wR2 = 0.1055

C40H32F6Fe3N4O6 946.25 monoclinic P21/n 10.8588(8) 9.2461(7) 18.2394(14) 90.00 95.4850(10) 90.00 1822.9(2) 2 1.724 1.265 960

C19H15F3FeN2O3S 464.24 monoclinic P21/c 17.0369(7) 11.8732(5) 20.1878(9) 90.00 114.7910(10) 90.00 3707.3(3) 8 1.664 0.980 1888

3

15219 5150 4478 474 0.980 0.0276 R1 = 0.0372 wR2 = 0.0888 R1 = 0.0441 wR2 = 0.0927

C52H34F24Fe5N4O18 1738.08 monoclinic P21/n 10.8059(10) 20.0299(19) 14.4707(14) 90.00 103.468(2) 90.00 3045.9(5) 2 1.895 1.311 1728

R1 = ∑||Fo| − |Fc||/∑|Fo| with Fo2 > 2σ(Fo2). wR2 = [∑w(|Fo2| − |Fc2|)2/∑|Fo2|2]1/2.

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−3) μ (mm−1) F(000) reflections collected independent observed [I >2σ(I)] no. of variables GOF Rint final R indices [I > 2σ(I)]a R indices (all data)a

2

1

4

24954 13191 6842 679 0.939 0.0401 R1 = 0.0466 wR2 = 0.0933 R1 = 0.1119 wR2 = 0.1201

C54H45Cl9Fe5N6 1376.26 triclinic P1̅ 10.5470(9) 17.1793(15) 17.3383(15) 60.4130(10) 89.053(2) 88.380(2) 2730.8(4) 2 1.674 1.781 1388

Table 3. Crystallographic Data and Pertinent Refinement Parameters for Compounds 1−7

10714 5641 3987 352 1.026 0.0341 R1 = 0.0509 wR2 = 0.1085 R1 = 0.0804 wR2 = 0.1265

C48H32Cl2Fe4N10O14 1267.14 triclinic P1̅ 10.0600(10) 10.2769(11) 12.5815(13) 98.906(2) 106.418(2) 101.995(2) 1188.4(2) 1 1.771 1.392 640

5

5863 3866 3400 372 1.046 0.0186 R1 = 0.0368 wR2 = 0.0933 R1 = 0.0430 wR2 = 0.0972

C27H19F3FeN4O3S 592.37 triclinic P1̅ 9.0818(10) 11.8202(14) 12.4100(14) 117.740(2) 98.255(2) 93.425(2) 1154.6(2) 2 1.704 0.810 604

6

25629 13617 8270 847 1.060 0.0541 R1 = 0.0668 wR2 = 0.1449 R1 = 0.1179 wR2 = 0.1911

C56H40F12Fe2N8O12S4 1484.90 triclinic P1̅ 10.205(3) 13.222(4) 22.065(6) 82.369(5) 87.672(5) 75.108(5) 2851.7(13) 2 1.729 0.767 1504

7

Inorganic Chemistry Article


Inorganic Chemistry

Article

intermolecular π−π stacking interactions (NP···NP and Cp···NP) allow the formation of the stacked dimer in the solid-state.

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1 inside an 8 mm o.d. vacuum-sealed glass tube. Anal. Calcd for C19H15N2Fe1F3SO3: C, 49.16; H, 3.26; N, 6.03. Found: C, 49.28; H, 3.21; N, 6.12. IR (KBr, cm−1): ν (FcNP) 1627(s), 1572(s), 1504(s), 1436(m), 1384(s), 1290 (w), 836 (m), 496 (m); ν (OTf−) 1240(vs), ν (NH) 3095 (br, m). 1H NMR (CD3CN, δ): 8.94 (s, br, Ha), 8.76 (s, br, Hc), 8.49 (s, br, Hd), 7.84 (s, br, He,b), 5.46(s, br, Hα), 5.05(s, br, Hβ), 4.33 (s, br, Hcp). ESI-MS: m/z 315 [M]+. Synthesis of [Fe(O2CCF3)2(FcNP)2(H2O)2] (2). To a methanolic solution (15 mL) of FcNP (94 mg, 0.3 mmol) was added a 0.3 mL 1(M) methanolic solution of trifluoroacetic acid (0.3 mmol), and the mixture was stirred for 12 h at room temperature. The resulting purple solution was concentrated under vacuum, and 15 mL of diethyl ether was added with stirring to induce precipitation. The red solid residue was filtered, washed with diethyl ether (3 × 10 mL), and dried under vacuum to afford 2. Yield: 60 mg (63%). X-ray-quality crystals were grown by layering petroleum ether onto a dichloromethane solution of 2 inside an 8 mm o.d. vacuum-sealed glass tube. Anal. Calcd for C40H32F6Fe3N4O6: C, 50.77; H, 3.41; N, 5.92. Found: C, 50.53; H, 3.52; N, 6.02. IR (KBr, cm−1): ν (FcNP) 1610(s), 1555(s), 1511(s), 1443(m), 1383(w), 918 (m), 837 (m), 783 (w); ν (CF3CO2) 1664(vs), 1202(s), 1180(vs); ν (H2O), 3429(m). Synthesis of [Fe3(OH2)2(O2CCF3)8(FcNP·H)2] (3). The reaction of FcNP (90 mg, 0.29 mmol) and 1(M) methanolic solution of trifluoroacetic acid (3 mL, 3 mmol) was carried out for 12 h following a procedure similar to that described for the synthesis of 2. Yield: 70 mg (70%). Single crystals were grown by the diffusion of petroleum ether onto a dichloromethane solution of 3 inside an 8 mm o.d. vacuum-sealed glass tube. Anal. Calcd for C52H34N4O18F24Fe5: C, 35.93; H, 1.97; N, 3.22. Found: C, 36.02; H, 1.88; N, 3.27. FT-IR (KBr, cm−1): ν (FcNP) 1613(m), 1561(m), 1510(m), 1445(w), 1388(w), 718(w), 473(m); ν (CF3CO2−) 1701(vs), 1204(vs), 1150(s); ν (H2O) 3423(m); ν (NH) 3141 (m). Synthesis of [(FcNP·H)3(Cl)][FeCl4]2 (4). The reaction of FcNP (85 mg, 0.27 mmol) and 1(M) HCl in Et2O (0.4 mL, 0.4 mmol) was carried out for 12 h following a procedure similar to that described for the synthesis of 1. Yield: 53 mg (72%). Single crystals were grown by the diffusion of petroleum ether onto a dichloromethane solution of the 4 inside an 8 mm o.d. vacuum-sealed glass tube. Anal. Calcd for C54H45N6Cl9Fe5: C, 47.13; H, 3.30; N, 6.11. Found: C, 47.22; H, 3.18; N, 6.21. IR (KBr, cm−1): ν (FcNP) 1618(m), 1556(m), 1512(m), 1443(w), 1390(w), 714(w), 471(m). Synthesis of [Fe2(Cl)2(FcNP)2(Pic)2] (5). To a methanolic solution (15 mL) of FcNP (100 mg, 0.32 mmol) was added a methanolic solution of picric acid (87 mg, 0.38 mmol), and the resulting dark green solution was stirred for 12 h at room temperature. The solution was then concentrated, and 15 mL of diethyl ether was added with stirring to induce precipitation. The resulting green solid residue was filtered and washed with diethyl ether (3 × 10 mL). Finally, it was dried in a desiccator with CaCl2 as desiccant to afford 5. Yield: 65 mg (64%). Single crystals were grown by diffusion of petroleum ether onto a dichloromethane solution of 5 inside an 8 mm o.d. vacuum-sealed glass tube. Anal. Calcd for C48H32Cl2Fe4N10O14: C, 45.50; H, 2.55; N, 11.05. Found: C, 45.37; H, 2.64; N, 11.13. Synthesis of [{FcNP2·H}2{CF3SO3}2] (6) and [FcNP2·H2][CF3SO3]2 (7). To a methanolic solution (15 mL) of FcNP2 (124 mg, 0.28 mmol) was added a 0.3 mL 1(M) methanolic solution of triflic acid (0.3 mmol) with stirring. Immediately, the mixture turned blue, and the resulting blue solution was stirred for an hour at room temperature. Then the solution was concentrated, and 15 mL of diethyl ether was added with stirring to induce precipitation. The resulting blue solid residue was washed with diethyl ether (3 × 10 mL) and dried in a vacuum to afford a blue solid. Yield: 150 mg (90%). Single crystals of 6 were grown by the diffusion of petroleum ether onto a dichloromethane solution of the blue solid inside an 8 mm o.d. vacuum-sealed glass tube. Needle type crystals of 6 were collected, and the mother liquor was kept in a refrigerator. After 3 weeks, a few block shaped crystals of 7 were harvested. Yield: 8 mg (4%). For compound 6: Anal. Calcd for C27H19N4F3SO3Fe1: C, 54.75; H, 3.23; N, 9.46. Found: C, 54.82; H, 3.28; N, 9.38. IR (KBr, cm−1): ν (OTf−) 1246 (vs), 1027(s); ν (FcNP2) 1630 (m), 1611 (m), 1575 (m), 1505 (w),

EXPERIMENTAL SECTION

General Procedures and Materials. All reactions were carried out under an inert nitrogen atmosphere with the use of standard Schlenk-line techniques. Solvents were dried by conventional methods, distilled under nitrogen, and deoxygenated prior to use. FcNP and FcNP2 were prepared by following literature procedures.38 Physical Measurements. Elemental analyses were carried out using a Thermoquest CE instruments model EA/110 CHNS-O elemental analyzer. ESI-MS spectra were recorded on a Waters Micromass Quattro Micro triple-quadrupole mass spectrometer. The 1H NMR spectra were obtained on a JEOL JNM-LAMBDA 500 MHz spectrophotometer. 1H NMR chemical shifts were referenced to the residual hydrogen signal of the deuterated solvents. Infrared spectra were recorded on a Bruker Vertex 70 FTIR spectrophotometer in the ranges from 400 to 4000 cm−1 using KBr pellets. UV−visible spectra were recorded using a Jasco V-670 UV/vis absorption spectrophotometer. Cyclic voltammetric studies were performed on a BAS Epsilon electrochemical workstation in propylene carbonate with 0.1 M tetra-n-butylammonium hexafluorophosphate(TBAPF6) as the supporting electrolyte. The working electrode was a BAS Pt disk electrode. The reference electrode was Ag/AgCl, and the auxiliary electrode was a Pt wire. The ferrocene/ferrocenium couple occurs at E1/2= +0.47 (60) V versus Ag/AgCl under the same experimental conditions. The potentials are reported in volts; the ΔE (Ep,a − Ep,c) values are in millivolts at a scan rate of 100 mV s−1. X-ray Data Collection and Refinement. . Single-crystal X-ray studies were performed on a CCD Bruker SMART APEX diffractometer equipped with an Oxford Instruments low-temperature attachment. All data were collected at 100(2) K using graphite monochromated Mo Kα radiation (λα) 0.71073 (Å). The frames were indexed, integrated, and scaled using the SMART and SAINT software packages,39 and the data were corrected for absorption using the SADABS programs.40 Structures were solved and refined with the SHELX suite of programs41 as implemented in X-Seed,42 while additional crystallographic calculations were performed by the program PLATON.43 The figures were drawn using ORTEP.44 CCDC nos. 872201−872207 contain the supplementary crystallographic data for compounds 1−7. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. The N−H hydrogens and water hydrogens were located in a difference Fourier map. The remaining hydrogen atoms were included at geometrically calculated positions in the final stages of the refinement and were refined according to the “riding model”. All non-hydrogen atoms were refined with anisotropic thermal parameters unless mentioned otherwise. Pertinent crystallographic data for compounds 1−7 are summarized in Table 3. Computational Details. Full geometry optimization calculations were performed at the DFT/B97D level of theory.33 Calculations were carried out by means of the G09 package.45 The double-ζ basis set of Hay and Wadt (LanL2DZ) with effective core potential (ECP) was used for Fe.46 The remaining atoms (H, C, and N) were described using 6-311++G(d,p) basis sets.47 Syntheses. Caution! Picrate salts are potentially explosive. Only small amounts of compound were synthesized and handled with extreme care under dark condition and low temperature. Synthesis of [FcNP·H][OTf] (1). To a methanolic solution (15 mL) of FcNP (150 mg, 0.48 mmol) was added a 0.5 mL 1(M) methanolic solution of HOTf (0.5 mmol) dropwise with stirring. The mixture immediately turned purple. After stirring for 1 h at room temperature, the resulting purple solution was concentrated to a small volume under vacuum, and 15 mL of diethyl ether was added with stirring to induce precipitation. The solid residue was then filtered and washed with diethyl ether (3 × 10 mL). Finally, it was dried under vacuum to afford 1. Yield: 184 mg (83%). X-ray-quality crystals were grown by layering petroleum ether onto a dichloromethane solution of 1440


Inorganic Chemistry

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1436 (m), 1387 (m), 843 (m); ν (N−H) 3085 (m). 1H NMR (CD3CN, δ): 8.84 (br,s, Ha), 8.65 (d, Hc, Jb,c = 6.1 Hz), 8.15 (d, Hd, Jd,e = 8 Hz), 7.84 (br,m, Hb), 7.65 (d, He, Jd,e = 7.2 Hz), 5.29(s, Hα), 4.84 (s, Hβ). ESI-MS: m/z 443 [M]+. For compound 7: Anal. Calcd for C28H20N4O6S2F6Fe1: C, 45.30; H, 2.72; N, 7.55. Found: C, 45.12; H, 2.68; N, 7.49.

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(9) (a) Oton, F.; Espinosa, A.; Tarraga, A.; de Arellano, C. R.; Molina, P. Chem.Eur. J. 2007, 13, 5742. (b) Alfonso, M.; Espinosa, A.; Tarraga, A.; Molina, P. Chem. Commun. 2012, 48, 6848. (10) (a) Neshvad, G.; Roberts, R. M. G.; Silver, J. J. Organomet. Chem. 1982, 236, 349. (b) Olah, G. A.; Mo, Y. K. J. Organomet. Chem. 1973, 60, 311. (11) Šarić, A.; Vrček, V.; Bühl, M. Organometallics 2008, 27, 394. (12) (a) Togni, A., Hayashi, T., Eds.; Ferrocenes; VCH Publishers: Weinheim, Germany, 1995. (b) Braga, D.; Polito, M.; Bracaccini, M.; D’Addario, D.; Tagliavini, E.; Proserpio, D. M.; Grepioni, F. Chem. Commun. 2002, 1080. (c) Chen-jie, F.; Chun-ying, D.; Dong, G.; Cheng, H.; Qing-jin, M.; Zheming, W.; Chun-hua, Y. Chem. Commun. 2001, 2540. (d) Horikoshi, R.; Nambu, C.; Mochida, T. Inorg. Chem. 2003, 42, 6868. (e) Zhu, Y.; Wolf, M. O. J. Am. Chem. Soc. 2000, 122, 10121. (f) Gao, Y.; Twamley, B.; Shreeve, J. M. Organometallics 2006, 25, 3364. (g) Reger, D. L.; Brown, K. J.; Gardinier, J. R.; Smith, M. D. Organometallics 2003, 22, 4973. (h) Guo, S. L.; Peters, F.; Fabrizi de Biani, F.; Bats, J. W.; Herdtweck, E.; Zanello, P.; Wagner, M. Inorg. Chem. 2001, 40, 4928. (i) Caballero, A.; Lloveras, V.; Curiel, D.; Tàrraga, A.; Espinosa, A.; Garcia, R.; Vidal-Gancedo, J.; Rovira, C.; Wurst, K.; Molina, P.; Veciana, J. Inorg. Chem. 2007, 46, 825. (j) Ion, A.; Moutet, J.-C.; Saint-Amam, E.; Royal, G.; Tingry, S.; Pecaut, J.; Menage, S.; Ziessel, R. Inorg. Chem. 2001, 40, 3632. (k) Buda, M.; Moutet, J.-C.; Saint-Aman, E.; Cian, A. D.; Fischer, J.; Ziessel, R. Inorg. Chem. 1998, 37, 4146. (l) Siemeling, U.; Brüggen, J. V. D.; Vorfeld, U.; Neumann, B.; Stammler, A.; Stamler, H.-G.; Brockhinke, A.; Plessow, R.; Zanello, P.; Laschi, F.; Biani, F. F. D.; Fontani, M.; Steenken, S.; Stapper, M.; Gurzadyn, G. Chem.Eur. J. 2003, 9, 2819. (m) Dong, T.-Y.; Chen, K.; Lin, M.-C.; Lee, L. Organometallics 2005, 24, 4198. (n) Irigoras, A.; Mercero, J. M.; Silanes, I.; Ugalde, J. M. J. Am. Chem. Soc. 2001, 123, 5040. (o) Zapata, F.; Caballero, A.; Espinosa, A.; Tárraga, A.; Molina, P. Inorg. Chem. 2009, 48, 11566. (p) Otón, F.; Ratera, I.; Espinosa, A.; Wurtz, K.; Parella, T.; Tárraga, A.; Veciana, J.; Molina, P. Chem.Eur. J. 2010, 16, 1532. (q) Singh, N.; Kumar, A.; Prasad, R.; Molloy, K. C.; Mahon, M. F. Dalton Trans. 2010, 39, 2667. (r) Zapata, F.; Caballero, A.; Espinosa, A.; Tárraga, A.; Molina, P. Dalton Trans. 2009, 3900. (s) Wei, K.; Ni, J.; Liu, Y. Inorg. Chem. 2010, 49, 1834. (t) Weiss, M.; Frey, W.; Peters, R. Organometallics 2012, 31, 6365. (u) Yuan, P.; Liu, S. H.; Xiong, W.; Yin, J.; Yu, G.; Sung, H. Y.; Williams, I. D.; Jia, G. Organometallics 2005, 24, 1452. (13) (a) Takemoto, S.; Kuwata, S.; Nishibayashi, Y.; Hidai, M. Inorg. Chem. 1998, 37, 6428. (b) Ramos, A.; Otten, E.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 15610. (c) Metallinos, C.; Tremblay, D.; Barrett, F. B.; Taylor, N. J. J. Organomet. Chem. 2006, 691, 2044. (d) Cui, X. L.; Delgado, R.; Costa, J.; Drew, M. G. B.; Costa, P. J.; Felix, V. Polyhedron 2010, 29, 1697. (e) Otón, F.; Espinosa, A.; Tárraga, A.; Molina, P. Organometallics 2007, 26, 6234. (f) Enders, M.; Kohl, G.; Pritzkow, H. Organometallics 2002, 21, 1111. (14) Enders, M.; Ludwig, G.; Pritzkow, H. Organometallics 2002, 21, 3856. (15) Trifan, D. S.; Nicholas, L. J. Am. Chem. Soc. 1957, 79, 2746. (16) (a) Lopez, J. L.; Tárraga, A.; Espinosa, A.; Velasco, M. D.; Molina, P.; Lloveras, V.; Gancedo, J. V.; Rovira, C.; Veciana, J.; Evans, D. J.; Wurst, K. Chem.Eur. J. 2004, 10, 1815. (b) Tárraga, A.; Molina, P.; López, J. L.; Espinosa, A. D.; Evans, J. Tetrahedron Lett. 2002, 43, 4717. (17) (a) Sadhukhan, N.; Bera, J. K. Inorg. Chem. 2009, 48, 978. (b) Sadhukhan, N.; Patra, S. K.; Sana, K.; Bera, J. K. Organometallics 2006, 25, 2914. (18) Patra, S. K.; Rahaman, S. M. W.; Majumdar, M.; Sinha, A.; Bera, J. K. Chem. Commun. 2008, 2511. (19) (a) Tshuva, E. Y.; Lippard, S. J. Chem. Rev. 2004, 104, 987. (b) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.; Lee, S.-K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.-S.; Zhou, J. Chem. Rev. 2000, 100, 235. (c) Kurtz, D. M., Jr. Chem. Rev. 1990, 90, 585. (d) Wallar, B. J.; Lipscomb, J. D. Chem. Rev. 1996, 96, 2625. (e) Du Bois, J.; Mizoguchi, T. J.; Lippard, S. J. Coord. Chem. Rev. 2000, 200− 202, 443.

ASSOCIATED CONTENT

S Supporting Information *

ORTEP diagrams for 1 and 4, NMR spectra for FcNP2 and protonated FcNP2, crystal packing diagram of 6, optimized geometries, energies, atomic coordinates for [(FcNP2·H)]+ and {(FcNP2·H)2}2+ are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +91-512-2597436. Tel: + 91-512-2597336. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank an anonymous reviewer for insightful comments and suggestions. Helpful discussions with Prof. Shridhar R. Gadre on the computational work are greatly appreciated. This work is financially supported by the Department of Science and Technology (DST), India, and Council of Scientific and Industrial Research (CSIR), India. J.K.B. thanks DST for the Swarnajayanti fellowship. M.S. thanks CSIR and T.G thanks UGC, India, for fellowships.

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DEDICATION Dedicated to Professor R. N. Mukherjee on the occasion of his 60th birthday.

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REFERENCES

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